Separators contribute materially to the performance, safety, and cost of lithium-ion batteries. During normal operation, a principal function of the separator is to prevent electronic conduction (i.e., short circuits or direct contact) between the anode and cathode while permitting ionic conduction via the electrolyte. For small commercial cells under abuse conditions, such as external short circuit or overcharge, the separator is required to shutdown at temperatures well below the temperature at which thermal runaway can occur. Shutdown results from collapse of the pores of the separator as a consequence of melting and viscous flow of polymer material of which the separator is made. Pore collapse slows down or stops ion flow between the electrodes. Nearly all lithium-ion battery separators contain polyethylene as part of a single- or multi-layer construction so that shutdown begins at about 130° C., which is the melting point of polyethylene.
Separators for the lithium-ion energy storage device market are presently manufactured from “dry” or “wet” processes. Celgard LLC and others have described a dry process in which polypropylene (PP) or polyethylene (PE) is extruded into a thin sheet and subjected to rapid drawdown. The sheet is then annealed at 10-25° C. below the polymer melting point such that crystallite size and orientation are controlled. The sheet is thereafter rapidly stretched in the machine direction (MD) to achieve slit-like pores or voids. Trilayer PP/PE/PP separators produced by the dry process are commonly used in lithium-ion rechargeable batteries.
Wet process separators composed of high molecular weight polyethylene are produced by extrusion of a plasticizer/polymer mixture at elevated temperature, followed by phase separation, biaxial stretching, and extraction of a pore-former material (i.e., plasticizer). The resultant separators have elliptical or spherical pores with good mechanical properties in the machine direction (MD) and transverse direction (TD). PE-based separators manufactured in accordance with wet process techniques by Tonen, Asahi, SK, and ENTEK Membranes LLC have found wide use in lithium-ion batteries.
More recently, battery failures occurring in commercial use have demonstrated that shutdown is not a guarantee of safety. The principal reason is that, after battery separator shutdown, residual stress and reduced mechanical properties above the polymer melting point can lead to shrinkage of, tearing of, or pinhole formation in the separator. The exposed electrodes can then touch and create an internal short circuit that leads to more heating, thermal runaway, and explosion.
In the case of large format lithium-ion cells designed for hybrid electric vehicle (HEV) or plug-in hybrid electric vehicle (PHEV) applications, the benefits of separator shutdown have been openly questioned because it is difficult to guarantee a sufficient rate and uniformity of shutdown throughout the complete cell. As such, battery designers are expected to handle at the system level failure modes that might involve separator shutdown. For example, external short circuits can be prevented by mechanical design and location within the vehicle. Overcharge, overdischarge, and high rate discharge are controlled by a Battery Management System (BMS). Thermal protection can also be handled on a system level, with one or both of built-in active and passive cooling systems. Another consideration is that these batteries are assembled as high voltage stacks, in which the shutdown of a single cell can itself create problems if, for instance, the shut down cell is driven into voltage reversal by the other cells in an electrical series string.
Many companies are, therefore, focused on modifying the construction of a lithium-ion battery to include (1) a heat-resistant separator or (2) a heat-resistant layer coated on either the electrodes or a conventional polyolefin separator. Heat-resistant separators composed of high temperature polymers (e.g., polyphenylene sulfide) have been produced on a limited basis from solution casting, electrospinning, or other process technologies. In these cases, the high polymer melting point prevents separator shutdown at temperatures below 200° C.
U.S. Pat. No. 7,638,230 B2 describes coating onto the negative electrode a porous heat resistant layer composed of an inorganic filler and a polymer binder. Inorganic fillers included magnesia, titantia, zirconia, alumina, or silica. Polymer binders included polyvinylidene fluoride and a modified rubber mixture containing acrylonitrile units. The heat resistant layer comprised 1-5 parts binder for every 100 parts inorganic filler by weight. Higher binder contents negatively impacted the high rate discharge characteristics of the battery. Moreover, the thickness of the porous heat-resistant layer had to be limited to 1-10 μm to achieve high discharge rates.
U.S. Patent Application Pub. Nos. US 2008/0292968 A1 and US 2009/0111025 A1 describe an organic/inorganic composite separator, in which a porous substrate is coated with a mixture of inorganic particles and one of a variety of polymer binders to form an active layer on at least one surface of the porous substrate. The porous substrate can be a nonwoven fabric, membrane, or polyolefin-based separator. Inorganic particles are selected from a group consisting of those that exhibit one or more of the following: dielectric constant greater than 5, piezoelectricity, and lithium ion conductivity. The composite separator purportedly exhibits excellent thermal safety, dimensional stability, electrochemical safety and lithium ion conductivity, and a high degree of swelling with electrolyte, compared to uncoated polyolefin-based separators used in lithium-ion batteries.
Evonik (Dresden, Germany) has produced heat-resistant separators by coating a porous ceramic layer of inorganic binder sol on each side of a polyester nonwoven membrane. While having excellent thermal stability, the membranes had extremely low mechanical integrity (e.g., tensile strain <10%), which created problems during battery assembly. The inorganic particles were also found to easily shed from the separator surface.
In each of the above approaches, an inorganic material-filled layer is applied in a secondary coating operation onto an electrode or porous substrate to provide heat resistance and prevent internal short circuits in the battery under high temperature, abuse conditions. The inorganic filled layer is applied as a coating because the described compositions do not provide sufficient mechanical integrity to form a freestanding porous sheet or film. “Freestanding” refers to a film having sufficient mechanical properties that permit manipulation such as winding and unwinding in film form for use in an energy storage device assembly.
The above limitations have motivated this invention of a freestanding, microporous, ultrahigh molecular weight polyethylene (UHMWPE)-based film that contains sufficient inorganic filler particles to provide low shrinkage while maintaining high porosity at temperatures above the melting point of the polymer matrix (>135° C.). Such freestanding, heat resistant films can be used alone or in combination with conventional polyolefin separators to prevent internal short circuits in energy storage devices such as lithium-ion batteries.